Physics for Kids

Theory of Relativity - Light and Time

One of the important concepts of the theory of relativity is the speed of light. You may think that light is instantaneous, that when you turn on the light in your room, the light instantly illuminates your entire room. However, it just seems that way, light actually travels at a given speed. That speed is just really fast.

The speed of light is 299,792,458 m/s. To put it another way, light travels 186,000 miles in a second. The speed of light is always the same. In physics, it is a constant and is usually represented by the letter "c" in formulas.

It turns out that there is nothing faster than light. It was this constant speed of light that inspired Albert Einstein to come up with the theory of relativity. Einstein found that when objects started to approach the speed of light, then the laws of physics that Isaac Newton came up with started to break down. Einstein's theory of relativity helped to explain why this happened.

Light Year

Astronomers use the speed of light to measure how far away stars and other galaxies are from Earth. A light year is the distance that light travels in one year. It is equal to 5,900,000,000,000 miles. That is five trillion, nine hundred billion miles.

Some stars and galaxies are so far away that the light we are seeing left the stars billions of years ago. Light can not only travel fast, but it can travel great distances too.

Time Dilation

One of the most interesting results of the theory of relativity is time dilation. Time dilation says that time will pass slower for someone traveling near the speed of light relative to someone standing still.

As an example of time dilation, let's imagine you left on a space flight today. On that space flight you flew around outer space at near the speed of light for around three years according to your clock. However, the clock on Earth was moving much faster. In fact, more than sixty years would have passed on Earth. You would only be a few years older, but everyone on Earth would be much older.

Time and Gravity

In Einstein's theory of general relativity he proposes that time will run more slowly the stronger the gravity. This means that time will pass faster on top of a mountain than at the beach. You will actually age slower at the beach than at the mountains. This is very small difference, however, nothing you would ever notice.

Light and Gravity

Another concept from general relativity is that rays of light will bend in a gravitational field. The Sun actually bends light enough that we can see stars that are barely behind the Sun. However, it takes a huge amount of gravity to bend light significantly. As big as the Earth is, it hardly bends light at all.

Is this science fiction?

A lot of the theory of relativity seems like science fiction, especially concepts like time dilation. However, scientists have been able to run experiments with very accurate clocks to prove the theory true. This is not science fiction, but real stuff!

Interesting Facts about the Theory of Relativity

Theoretically, if you could travel faster than the speed of light, time would go in reverse.

Scientists think that the universe is expanding and that parts of it are moving away from us at faster than the speed of light.

Albert Einstein once said, "When you are courting a nice girl an hour seems like a second. When you sit on a red-hot cinder a second seems like an hour. That's relativity."

Many aspects of the theory of relativity have been confirmed by experiments run with particle accelerators. Particle accelerators can accelerate electrons and protons to nearly the speed of light.